1. Field of the Invention
The present invention generally relates to a brake apparatus for a vehicle and, more particularly, to a brake apparatus suitable for an electric vehicle.
2. Description of the Related Art
In an electric vehicle, braking is performed normally by using both a hydraulic brake and a regenerative brake. In the hydraulic brake, a hydraulic pressure is provided to a wheel cylinder of each of the wheels so as to generate a brake torque in response to a braking operation by a driver. Whereas, in the regenerative brake, a brake torque is generated by a regeneration action obtained based on a reverse electromotive force generated in a traction motor which drives the drive wheels of a vehicle. The regenerative brake regenerates electric energy which is to be charged to a battery based on the reverse electromotive force generated by inertial rotation of the traction motor. The electric energy (hereinafter referred to as regenerative energy) regenerated by the regenerative brake varies in response to a regenerative brake torque. It is preferable to increase the regenerative energy so as to maintain a battery in a sufficiently charged state. Thus, in order to maximize the regenerative energy, it is desired to maximize the regenerative brake torque within a range in which the regenerative energy is acceptable for the battery. If the regenerative brake torque is not sufficient for the required brake torque determined by a braking operation performed by a driver, a hydraulic pressure which generates a brake torque corresponding to the shortage of the brake torque is provided to the wheel cylinder so as to achieve the required brake torque.
Japanese Laid-Open Patent Application No. 7-205800 discloses a brake apparatus for an electric vehicle having the above-mentioned function. In this brake apparatus, a hydraulic pressure control valve provides a proportionally reduced master cylinder pressure to both front wheels and rear wheels. That is, the hydraulic pressure control valve provides to each wheel cylinder a pressure corresponding to the difference between the maximum acceptable regenerative torque and the required brake torque.
The required brake torque for a vehicle and the maximum regenerative brake torque are not always equal between the right side wheels and the left side wheels. That is, for example, the required brake torque for the entire vehicle is distributed to each of the wheels as required wheel brake torque in response to a moving condition such as turning a curve or moving straight or when a load is applied to each of the wheels. Accordingly, the required wheel brake torque may differ wheel to wheel. Additionally, in a case where a traction motor is separately provided to each of the wheels, the maximum regenerative brake torque may also differ from wheel to wheel due to variation in the operational conditions of each of the traction motors. In such a case, the wheel cylinder pressure provided to each of the wheel cylinders must be separately controlled so as to provide the required wheel brake torque to each of the wheels in a state where the regenerative torque is to be maintained at a maximum.
However, the conventional brake apparatus provided in the above-mentioned electric vehicle has a single wheel cylinder for each of the front wheels and the rear wheels. Thus, the brake torque applied to the left wheels and right wheels cannot be controlled separately. Accordingly, in the conventional brake apparatus, a hydraulic pressure corresponding to a larger torque difference is applied to the wheel cylinder connected to both the left wheel and the right wheel in either the front side or the rear side so as to achieve the required brake torque on the side where the larger torque difference is generated. In this case, on the side where the required wheel brake torque is smaller, the regenerative brake torque must be reduced so that the total brake torque does not exceed the maximum allowable torque. Accordingly, in the conventional brake apparatus for the electric vehicle, there is a problem in that the regenerative energy cannot be maintained at the maximum.